Attention has been paid to an excimer laser device which is usable as a light source for an unit for projecting and exposing figures in a reduced size for the purpose of producing semiconductor devices. This is attributable to many remarkable advantages derived from the facts that it is possible to extend a limit of light exposure to a level of wavelength of excimer laser shorter than 0.5 micron because an excimer laser has a short wavelength (e.g., KrF laser has a wavelength of about 248.4 nm). the excimer laser device has a deep focus depth compared with a g line and an i line emitted from a mercury lamp which has been heretofore used, provided that each device has a same resolution, the number of lens apertures (NA) can be reduced, an exposing range can be widened and large power can be generated by the device.
However, the excimer laser device has two significant problems to be solved when it is used as a light source for an unit for projecting and exposing figures in a reduced size.
One of them is that the excimer laser has a short wavelength of 248.35 nm and only quartz can be used as a starting material for lenses in view of uniformity and machining accuracy due to the fact that material through which it can transmit is limited only to quartz, CaF.sub.2 and MgF.sub.2. This makes it impossible to design a lens for projecting figures in a reduced size with its color aberration corrected. Therefore, there is a need of narrowing an operative range of the excimer laser device to such an extent that any color aberration can be neglected.
The other problem to be solved is how to prevent a speckle pattern from appearing as the operative range of the excimer laser device is narrowed and how to suppress reduction of power caused as the operative range of the excimer laser device is narrowed.
As an prior technique of narrowing the operative range of the excimer laser device, there is known a so-called injection lock process. This injection lock process is such that a wavelength selection element (etalon, diffraction grating, prism or the like) is disposed in a cavity at an oscillator stage, laser light is oscillated in a single mode while a spatial mode is restricted by a pin hole, and the thus oscillated laser light is synchronously introduced into the excimer laser device via an amplifying stage. This arrangement allows laser light outputted from the excimer laser device to have a high property of coherence. Thus, when the excimer laser device operable in accordance with the injection lock process is used as a light source for an unit for projecting and exposing figures in a reduced size, a speckle pattern is generated. In general, it has been considered that generation of a speckle pattern depends on the number of spatial lateral modes involved in laser light. Further, it has been known that when the spatial lateral modes involved in the laser light has a small number, a speckle pattern is liable to appear and that, to the contrary, when the number of spatial lateral modes increase, the speckle pattern hardly appears. The above-described injection lock process is substantially concerned with a technique for narrowing the operative range of the excimer laser device by largely reducing the number of spatial lateral modes. However, since the injection lock process has a significant problem of generation of a speckle pattern, the excimer laser device can not be employed for an unit for projecting and exposing figures in a reduced size.
In addition, as other promising technique for narrowing the operative range of the excimer laser device, there is known a technique of using etalons each serving as a wavelength selecting element. In this connection, it will be helpful to note that as a prior technique of using etalons in that way, AT & T Bell Research Institute has proposed a technique for narrowing the operative range of the excimer laser device by arranging an etalon between a front mirror and a laser chamber for the excimer laser device. With this technique, however, it has been found that it has a problem that a large quantity of power loss is caused by inserting the etalon between the front mirror and the laser chamber and moreover it has a drawback that the number of spatial lateral modes can not be increased so much.
In view of the aforementioned problem and drawback, the inventors conducted a variety of research and development works and reached a knowledge that excimer laser light can be generated in the narrowed operative range with an output of about 50 mJ per each pulse by arranging etalons each having a large effective diameter (about several decimal millimeters in diameter) between a rear mirror and a laser chamber for the excimer laser device to uniformly narrow the operative range of the excimer laser device of which spectrum width is less than about 0.003 nm in terms of a half value across the full width within the range of 20.times.10 mmb. Namely, the inventors resolved an essential problem which arose when the excimer laser device was used as a light source for an unit for projecting and exposing figures in a reduced size, by arranging etalons between the rear mirror and the laser chamber for the excimer laser device with the operative range narrowed, the number of spatial lateral modes maintained and the power loss reduced by arrangement of the etalons in that way.
In fact, arrangement of the etalons between the rear mirror and the laser chamber for the excimer laser device provides excellent advantages that the operative range can be narrowed, the number of spatial lateral modes can be maintained and the power loss can be reduced. However, it has been found that such arrangement has problems that physical variation, e.g., variation of a temperature of each etalon or the like occurs due to remarkable increased power of laser light transmitting through the etalons, resulting in variation of center wavelength of oscillated output laser light, oscillation of plural wavelengths and substantially reduced power of laser light. This tendency is notable particularly when two or more etalons each having a different free spectrum range are used to narrow the operative range of an excimer laser device.
As is well known, the excimer laser is gas laser. With the excimer gas, however, it is known that there appears a phenomenon that a performance of laser medium gas for the excimer laser is gradually degraded and laser power is reduced as time elapses. To prevent such phenomenon from appearing, output control is carried out for holding a laser output constant by controlling an intensity of excitation of the laser medium, i.e., discharge voltage. In detail, to hold the laser output constant, the output control is practiced by varying a voltage between electrodes to vary an intensity of excitation of the laser medium or by carrying out partial replacement of gas serving as a laser medium via a gas controller.
The excimer laser device is a laser device from which pulses are oscillated, and energy of each pulse fluctuates to some extent. Accordingly, when variation of laser power is to be monitored, a plurality of pulses are sequentially monitored to evaluate laser power by averaging the sampled pulses.
In a case of superposition control for allowing a wavelength of laser light transmitting through each etalon to coincide to each other, center wavelength power is monitored. Also in a case of excitation intensity control, laser power is monitored. These monitoring operations are performed to evaluate laser power by sampling a plurality of pulses and then averaging the sampled pulses.
In a case where the both superposition control and excitation intensity control are used together, since a superposition control system itself varies laser power and determines that the time point when center wavelength power is maximized coincides to the time point when the superposition control expires, there is a danger that incorrect determination is made such that the superposition wavelength control expires in spite of the fact that center wavelength power is practically maximized by an excitation intensity control system. This is same with the excitation intensity control. Namely, there is a danger that incorrect determination is made such that the excitation intensity control expires in spite of the fact that center wavelength power is practically maximized by the superposition control system.
Thus, when the superposition control and the excitation intensity control are used together, it can not be discriminated whether reduction of laser power is caused due to incorrect superposition of etalons or deterioration of a performance of the laser medium. As a result, there arises a danger that wavelength and laser power can not be controlled with stability.